There seems to be a small community of DIY vehicle builders, but they tend to stick to forums and such. I think because of liability issues (and rules for cars are pretty strict as they are), there isn't a lot of sharing between builders beyond the basic frame
I don't know how reliable things like CANpie or Can4linux are (Controller Area Network or CAN used for internal systems communication in vehicles, and these are open source)
Speaking of vehicles, one design I'd like to see, even just for farms or for private property use only, is the "Rocker Bogie" design. It's the same system used by pretty much all NASA rovers since Sojourner
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